Hydrogen storage units utilising metal hydrides such as catalysed MgH2 require temperatures above 280° C. to effect a positive pressure desorption. The heat loss from a heated well insulated solid state storage cylinder with the dimensions of a commonly used G sized compressed gas cylinder can approximate 500 watts. Therefore, the heat loss from a heated 16 cylinder manifolded solid state pack can approximate 8 kilowatts. This 8 kilowatts is additional to the energy required to break the MH-hydrogen bonds and affect the adsorption. Hence the resulting thermal efficiency of such a system is extremely low resulting in increased electricity usage and poor carbon footprint.
As each storage vessel requires significant heat input to desorb hydrogen, it is advantageous to heat one vessel at a time (1) to reduce the total heating power requirement at start-up or (2) enable the desorption of hydrogen to occur at a much faster rate when a fixed amount of heating power is available. Hence, the applicant is pursuing the concept of a manifolded storage system including a multiple number of hydrogen storage vessels where only one cylinder is desorbing at a time.
Unlike a compressed gas storage unit, a solid state hydrogen storage unit containing hydrogen storage material empties under a constant pressure. In a compressed gas unit, the depth of discharge can be accurately inferred from the remaining gas pressure in the cylinder. In contrast, a solid state hydrogen storage unit will discharge from full to over 90% empty at a constant equilibrium pressure determined by the operating temperature. Once the volume of stored gas is too low to supply the flow for the required demand, the pressure in the hydrogen storage vessel will reduce quickly from the equilibrium point to zero. This is typically once the depth of discharge is beyond 90%.
Generally, when hydrogen is being desorbed from only a single vessel at a time, once the equilibrium pressure in that hydrogen storage vessel begins to drop, it is too late to start heating up the next cylinder in sequence as the time to bring the vessel to desorption pressure and temperature far exceeds the remaining supply capacity of the current near empty vessel. In order to provide a constant supply of hydrogen to meet a hydrogen demand, it is desirable that the next hydrogen storage vessel in sequence begins heating well before the constant equilibrium pressure begins to drop.
Additionally, once the active desorbing cylinder is empty, it is desirable to cool down the cylinder to minimise heat loss. However, since the hydriding/dehydriding reaction is a reversible reaction, the reaction will reverse and the hydrogen storage material will absorb hydrogen once the temperature of the hydrogen storage material drops. FIG. 1 shows the absorption rate of MgH2 as a function of temperature for a given pressure. For example, the plot of the absorption rate indicates that the reaction is in the desorption direction at temperatures above the equilibrium temperature. For temperatures below the equilibrium point the reaction is in the absorption direction. Therefore, if the cylinders are connected in parallel to a supply manifold and the next cylinder in the sequence is now supplying hydrogen by being heated to above the equilibrium temperature, the previous cylinder will absorb all of this hydrogen as it cools down below the equilibrium point leaving zero net hydrogen supply to meet the hydrogen demand.
It is desirable that the present invention provides a hydrogen storage system or supply arrangement which addresses one or more of the above problems.
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.